Visual acuity is a physiological factor of primary importance in determining qualifications for job placement, licenses, and the like. Most often it is determined through the use of the Snellen letter chart, in which the threshold letter size for the subject is found and converted to a visual acuity measurement. This procedure is subject to several subtle variables which can significantly affect the outcome, such as inappropriate room lighting, test lamp aging, failure of examinee to cover each eye properly, examiner recording error, and inherent testing pressures.
Furthermore, the results of such a visual acuity test are seriously affected by the desires of the subject. For example, in Armed Forces enlistment induction, drivers license testing, or in the case of children who do not wish to wear glasses, an individual with poor acuity may memorize the Snellen chart to achieve a "passing" score. On the other hand, those who wish to avoid Armed Forces draft induction may purposely misread the Snellen chart to score below the minimum acceptable visual acuity level.
Also, it is often difficult to ascertain the visual acuity of children, due to their unfamiliarity with the letters of the Snellen chart, or their unwillingness to cooperate with the examiner in the testing procedure. Failure to diagnose vision problems in children, due to such testing difficulties, may result in unexplained low learning achievement, exacerbated visual abnormalities, and other serious consequences.